1. Field of the Invention
The present invention relates generally to expandable tubular devices, and particularly to implantable medical devices for use in naturally occurring or surgically created vessels, ducts, or lumens in living beings, and more specifically to catheter-delivered endoluminal stent-graft prostheses, and methods of using such devices in, for instance, cardiovascular systems.
2. Description of Related Art
Endoluminal therapies are currently under investigation as alternative methods of treating vascular disease. These approaches involve the insertion of a prosthetic device into the vasculature through a small, often percutaneous, access site in a remote vessel, followed by the intraluminal delivery and deployment of a prosthesis via transcatheter techniques. In contrast to conventional surgical therapies, endoluminal treatments are distinguished by their xe2x80x9cminimally invasivexe2x80x9d nature.
Endoluminal therapies have evolved to address a variety of cardiovascular pathologies. Initial outcomes of these procedures, although preliminary, are encouraging. Not surprisingly, endoluminal therapies have generated intense interest within the vascular surgery and interventional radiology communities because these techniques have the potential to simplify the delivery of therapy, improve procedural outcomes, decrease procedural costs, and broaden the patient population that may benefit from intervention.
Endoluminal stent-grafts are catheter-deliverable endoluminal prostheses comprised of an intravascular stent component and a prosthetic graft component. The function of these devices is to provide a mechanically supported intraluminal conduit that enables blood flow through pathologic vascular segments without the need for open surgery.
The stent component of the stent-graft functions as an arterial attachment mechanism and provides structural support to both the graft and the treated vascular segment. By design, stents are delivered to the vasculature in a low profile, small diameter delivery configuration, and can be elastically or plastically expanded to a secondary, large diameter configuration upon deployment. Vascular attachment is achieved by an interference fit created when a stent is deployed within the lumen of a vessel having a diameter smaller than that of the enlarged diameter of the stent.
The graft component of the stent-graft is generally constructed from a biocompatible material, such as polytetrafluoroethylene (PTFE), expanded PTFE, woven polyester, or polyurethane. The graft component has a number of proven and theoretical functions, including: segregating potential thromboemboli or atheroemboli from the bloodstream, presenting a physical barrier to mass transport between the bloodstream and arterial wall, and mitigating cellular infiltration and the host inflammatory response.
Mechanical properties play an important role in determining the performance of an endoluminal stent-graft. Since the graft component typically lacks significant structural integrity, the mechanical behavior of the stent-graft predominantly depends upon the mechanical properties of the stent component. Stents are typically classified by the type of mechanism required to induce dilatation from the delivery (small diameter) configuration, to the deployed (large diameter) configuration. Self-expanding stents are designed to spontaneously dilate (such as, elastically recover) from the delivery diameter up to a maximum, pre-determined deployed diameter. Contrastly, balloon-expandable stents are designed to be plastically enlarged over a range of sizes with the use of appropriately sized and pressurized dilatation balloons or similar devices that apply distensive force. Consequently, self-expanding stents exert a continuous, radially-outward directed force on periluminal tissues, while balloon-expandable stents assume a fixed diameter that resists recoil of the surrounding periluminal tissues.
Both types of stents have useful features. For example, in comparison to balloon-expandable stents, self-expanding stents can be rapidly deployed without the use of dilatation balloons, are elastic and therefore less prone to permanent distortion from external compression (i.e., they are resistant to permanent or plastic deformation from external compression due to their ability to elastically recover from external loads). Self-expanding stents can also radially adapt to post-deployment vascular remodeling, and retain some of the natural compliance of the vascular tissues. Since the luminal diameter of self-expanding stents cannot be adjusted (i.e., enlarged) to any appreciable degree beyond their maximum manufactured diameter, accurate sizing of the host vessel is critical. A sizing mismatch resulting in significant oversizing can cause vascular trauma, overcompression of the host vascular tissue, and/or obstructive invagination of the stent into the lumen. Undersizing, in turn, can result in a poor interference fit, inadequate anchoring, device migration, and/or leakage of blood into the peri-stent compartment. In contrast, balloon-expandable stents are more versatile when it comes to conforming to irregular vascular morphologies because their diameter can be radially adjusted through a range of diameters. However, balloon-expandable stents are prone to undesirable plastic deformation if loaded externally, which can compromise luminal diameter and blood flow.
Accordingly, it is a primary purpose of the present invention to develop an endoluminal stent-graft that maintains some of the best qualities of both self-expanding and balloon expandable stents while avoiding major deficiencies of each.
This purpose and other purposes of the present invention will become evident from a review of the following specification.
The present invention combines both self-expanding and balloon-distensible properties into a single diametrically expandable stent-graft device. One embodiment of the stent-graft of the present invention is adapted to achieve three distinct phases in use. First, the device is radially constrained on or upon a delivery device to a first diametrical dimension for insertion into a vessel. Second, when unconstrained, the stent-graft expands to achieve a second diametrical dimension within the vessel. Third, the device diameter can then be further enlarged by application of a distensive force, such as through use of a balloon dilatation catheter or via controlled creep processes engineered into the device, to variable third diametrical dimensions to fit the dimensions of the vessel or adjust to changing dimension of the vessel. During this process, the circumferential length of the graft compresses or increases with device diameter thus allowing the luminal surface to remain essentially wrinkle free. Accordingly, the stent-graft of the present invention combines the properties of both self-expanding and balloon-expandable stents into a single, easy to use device.
In a first embodiment of the present invention, a balloon-distensible graft material, such as expanded polytetrafluoroethylene (expanded PTFE), is laminated to a radially-compressed, self-expanding stent frame. By balancing (a) the inherent nature of the graft material to resist stretching beyond its original dimensions, with (b) the outward pressure exerted by the self-expanding stent frame component, the self-expanding stent will automatically assume the desired second diametrical dimension when unconstrained from the delivery device. To achieve distention beyond this second diametrical dimension, the graft material is selected to deform to variable third diametrical dimensions with application of sufficient distensive force. Sufficient distensive force may be applied over relatively short periods of time (e.g., force applied by a balloon dilatation catheter) or lower forces applied over relatively long periods of time (e.g., force applied by the self-expanding stent in-situ resulting in radial creep distention of the graft component). Since the preferred stent component is self-expanding, its diameter will essentially match that of the graft material.
A second embodiment utilizes the same principles as described previously, except that the self-expanding device is maintained at its introductory profile using an integral constraining lamina. This embodiment is balloon-mounted and balloon-deployed yet retains the elastic properties of the self-expanding structural component. The self-expanding stent is attached to a balloon distensible graft component having a diameter essentially equal to the first functional diameter and is prevented from expanding by the graft component or integral constraining lamina (i.e., outward radial force exerted by the stent is less than the circumferential radial strength of the graft component or lamina). This device can then be mounted upon a balloon dilatation catheter. The device is delivered to the deployment site in a manner consistent with current delivery techniques and is deployed via inflation of the balloon. During inflation, the graft component or integral constraining lamina yields or is disrupted by the balloon (i.e., increasing pressure). Following yielding or disruption of graft component or the integral constraining lamina, the diameter of the device enlarges via continued balloon dilatation augmented by the continuous, radially directed forces exerted by the self-expanding stent. Since the graft component undergoes circumferential elongation proportional to the diameter of the device, a wrinkle free or otherwise essentially smooth luminal surface is maintained throughout all functional diameters.
After the appropriate diameter is achieved, the balloon can be deflated and the luminal surface remains smooth. The self-expanding stent exerts a continued outward radial force that is exerted on 1) the graft component and/or integral constraining lamina, (thereby preserving a smooth luminal surface) and 2) the vessel wall. With time, the lumen may narrow due to normal or pathological healing. To counteract this narrowing, the stent-graft can increase in circumferential length via creep processes resulting from the outward radial force supplied by the self-expanding stent while retaining a smooth luminal surface. Also, the stent-graft can be progressively balloon dilated to larger functional diameters up to the maximum manufactured diameter of the self-expanding stent.
The device of the present invention provides a number of distinct advantages over previous expandable stent-graft devices. For example, the mechanism of dilatation can be varied in one or more distinct regions along the length of device allowing a uniquely customized fit within the vessel. Further, the radial strength of the device can be varied in one or more distinct regions along the length of the device. Further, the device of the present invention provides the clinician with a range of essentially wrinkle-free functional diameters, and can be engineered to enlarge over time to compensate for the effects of luminal narrowing due to normal or pathological healing (herein referred to as xe2x80x9ccompensatory enlargementxe2x80x9d). Additionally, the device of the present invention provides its adjustable distension properties without undergoing significant shortening in length thus providing better control of device placement and deployment. These and other benefits of the present invention will be appreciated from review of the following descriptions.